When I cleaned up the engine bay of my wife’s 1990 XJ-S convertible (5.3L V12 with Marelli ignition) last year, I went a level deeper than the recent work I did on my 1990 V12 Vanden Plas (5.3L V12 with Lucas ignition) and I removed the intake manifolds, leaky cam shaft covers, and adjusted the valve clearances. I have adjusted the intake and exhaust valve clearances many times on my 4.2L XK engine equipped Jaguars but that was a first for me on a V12 and I learned to use the different tools and techniques. Most of the 24 valves were out of spec and took a while to adjust primarily because there were so many to adjust.
I did not remove the intake manifolds or cam shaft covers on my 1990 V12 Vanden Plas because it has much lower mileage and the cam covers were not leaking.
Other than that it was much the same work cleaning, painting, polishing, replacing most everything on the top of the engine. I also took the time to rewrap all the vinyl wrapped wiring harnesses with some 3M high temperature vinyl tape especially the ones where I removed and replaced the broken plastic connectors. I was pleasantly surprised how much rewrapping the harnesses improved the engine bay appearance.